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Newbie JBL L100 restoration and upgrade

Hi there,

Let me introduce myself. My name is Lee, I moved to Southern NJ from England about 10 years ago, bringing with me all my possesions, including my HiFi and my 1970 Ford Capri. I have always been into HiFi and speakers in particular, I grew up around a pair of the original Mission 700s that my dad had, and a pair of KEF 104aBs that my mom had. I remember my dad and I walking to HiFi shows at a big hotel that overlooked Blackheath. Even to my young ears, the sounds of a pair of Mission 770s, powered by matching Mission amps, fed by a Linn Sondek LP12 turntable was truly awesome.

I recently came across a pair of JBL L100s. The cabinets looked pretty good apart from the usual scratches and rotted foam grilles, the drivers were free from damage apart from a pushed in tweeter dome, so I bought them.

When I got them home and hooked them up, I was impressed by their sound, particularly their low frequency response. If you've ever heard the music of Level 42, the early stuff before they got all poppy, you'll know all about Mark King's slap-bass technique. Try looking on YouTube for a track called "43", not the live version, there's a part about 3 minutes in where I swore the JBLs were going to blow me through the wall into my back yard. My current speakers are what I call my "bass monsters", I built these 20 years ago with SEAs drivers and matching crossover, with a dome tweeter, 8" and 10" woofers (the 10" runs through a subwoofer crossover). Until now, I had yet to find a pair of speakers with as good a bass response. Comparing the two, the foward response of the JBLs became more apparent, it seemed like the midrange and treble was more "shouty" for want of a better word. I decided to restore them, but give them some upgrades at the same time. I wanted to try one of the different crossover designs out there, but at the same time be able to switch back to the original crossover if I wanted that vintage sound back. I also wanted to do things like add some decent binding posts on the rear, make some new grilles, maybe even relocate that one grille peg that everyone who's seen them asks why it doesn't line up with the others. If this was a car, it would be a restomod...

The first problem I found was one of the woofers would make a scraping sound if you tapped the cone ("don't tap it then" should be the answer). It seemed to work OK playing music, but I didn't want it to become a problem later on, so I took the woofer out. Well, that was the plan, but the woofer wouldn't come out at first, so I removed the midrange and tweeter to see if I could get access to the back. I managed to squeeze my hand through the tweeter hole and eventually get the woofer out by pushing on the frame edge with my fingers. It turns out the voice coil had become detached from the spider. What makes these woofers great is you can remove the magnet to make repairs, and once I got the magnet off, I found a date stamp inside of September 18 1974.

After removing the dust cap, I re-attached the voice coil to the spider using some black glue purchased from Orange County Speaker. After some trial and error, I found a piece of glossy cardboard that was part of a magazine cover to use as a shim between the coil and the magnet pole. Since the spider wasn't in contact with the coil like it should be, once I got the glue around the edge of the coil, I used a coffee mug to push down on the cone, which brought the coil back in contact with the spider. Once this was done, and given a few days to dry, I tested the woofer using some music with long, low bass sweeps, and it seems to work perfectly! I reattached the dust cap using the same glue, and then gave it a couple of coats of black "Wet Look" speaker paint from Parts Express. Looks good doesn't it?

I then turned my attention to the crossover. After looking around on the 'net, it looked like Dennis Murphy's crossover design was the way to go. I wanted to be able to switch back to the original spec crossover if I need to, so I modified the design to include three switches to switch between the Murphy and JBL spec crossovers. As you probably know, the midrange on this speaker (well, mine anyway, and it appears to be commonplace) is wired out of phase. The woofer and tweeter both have their negative terminals connected to the negative input, whereas the midrange has it's positive terminal connected to the negative input. I was looking at Parts Express, and they stock a heavy duty 4 pole Double Throw switch that I could use for the mid, and use two DPDT switches for the woofer and tweeter. Two of the poles on the midrange switch would be to switch between crossovers, the other would be to reverse the polarity when using the JBL crossover. I could have used a 6 pole switch for everything, but finding one with a high current rating was a problem. Not that I planned on doing this, but you could in theory listen to the speaker with the JBL crossover on the woofer, and the Murphy crossover on the midrange and tweeter, or any other combination. Here you can see my modified schematic. It doesn't include the L-pads, but these will be used on the JBL crossover section.

Using parts from Parts Express and Erse Audio, I was able to build my crossovers. I did try using a big 5x9" circuit board from Parts Express, but it proved too small, it was mainly the big 25w resistors I was using. So I used a piece of scrap hardwood I had lying around the house, and hard-wired everything on the back. My next plan is to remake the JBL crossovers with some new capacitors and L-pads. I will then mount a 5x7" dished plate on the back of the speaker cabinet which will hold some chunky binding posts, and the three switches to switch between crossovers.

Got to spend some more time working on the JBL L100 restoration and upgrade.

Here is a picture of my "re-manufactured" factory spec crossovers. Since the Parts Express L-pads have a longer shaft than the stock crossovers, I cut a new piece of 1/2" MDF and mounted them on there.

And here are the new terminal plates I made, again using parts from Parts Express. The middle switch is the 4-pole double throw for the midrange, I'm going to wire two poles to switch between the Murphy crossover and the JBL spec crossover, and the other two poles to switch the polarity of the midrange driver, just like JBL did.

I got one midrange switch wired up today, not mounted on the plate (see above) only another five to go. I did this first since they sit between the woofer and tweeter switches, and won't be as easy to get to when they're all together. I couldn't wait, I had to hear what the difference was, so I connected it up with wires twisted here and there.

What I can tell you is the difference between the Murphy crossover and the JBL is staggering. With the JBL, it sounds like the midrange is the tweeter as well, there is FAR too much treble getting to it. I've just posted a request on a Freecycle web site I'm on for a microphone. If I can get hold of one, I'll post a video up on YouTube of the differences between the two crossovers.

I mounted the new upgraded Murphy crossover board to the back panel, drilling the top holes with a Milwaukee 1/4 in right angle adapter I got from Home Depot (part number 48-32-2100) with a Craftsman drill bit with a 1/4 hex shank. I used some strips of foam insulation behind the board to stop it vibrating. You can see the new JBL spec crossover L-pads in the top of the picture, this was mounting using the orignal screw holes in the baffle (for the moment, I might fill these when I refinish the cabinets).

Here is a close-up of the terminal and switch plate. I tried to run every common or ground wire back to the negative terminal in a kind of star-earth way, that's why there are so many wires : 2 to the JBL spec crossovers, 3 to the Murphy crossover, 2 to the woofer and tweeter, 1 to the midrange switch.

I also replaced the old fibreglass insulation with some new acoustic wadding or cabinet damping.

Here is a view of the back panel, looks more business like now doesn't it?

And here they are fully assembled. I had to pull out one of the tweeter domes with a pin, so I sealed it using some "Wet Look" speaker paint from Parts Express. I did the woofer caps as well, since I had to remove one.

Before everything was buttoned up, I was able to play each driver on it's own, just to make sure I wired the switches the correct way. This revealed some interesting results...

The woofer reproduces a lot more midrange frequencies using the JBL crossover, these are rolled off with the Murphy. From the factory, it has no crossover components of any kind, relying on it's natural roll-off. Likewise, the tweeter reproduces more midrange frequencies with the JBL crossover than the Murphy. I hate to point the finger at the little guy in the middle, but I've already mentioned the midrange reproduces a lot more treble than it should in JBL crossover mode. What is also interesting is it seems to get more low frequencies as well. This was noticed using nothing more high tech than some dust that just happened to be sitting on the cone. With just this driver connected (hence no vibration from the woofer), switching from Murphy to JBL caused the dust to bounce off the cone. Actually, the speaker doesn't sound all that bad with just the woofer and tweeter connected using the JBL crossover, in a loudness button "boom and tizz" kind of way, less midrange.

Connecting all the drivers and placing them back in the cabinet, it is easy to hear the difference when switching crossovers. Going from high to low, and with the switches set to the Murphy crossover, here's what happens. Switching the tweeter over from the Murphy to the JBL crossover, now there's more midrange. Switching the midrange now there's more midrange and even more treble, switching the woofer over adds even more midrange.

It's easy to see why this speaker is impressive when you first hear it, there's so much attack because all three drivers are having a go.

What's weird is the midrange interaction with the woofer. I don't know what you call it, but we all know the sound a pair of out-of-phase speakers make. That weird sound like it's coming from behind you, or inside your head, it's kind of hard to describe. I've been to peoples houses (who should know better) and told them one of their speakers is wired back to front. Of course they deny it, then when they check, it is. I've been to HiFi shops, record shops, heard car systems, and can easily tell when a speaker is wired up wrong. Well, I had each of these L100s lying on their backs on the floor with a piece of wood propping them up so I could switch between crossovers. Switching from the Murphy to the JBL crossover on the midrange, with the woofer set on the JBL crossover, sure enough you can hear the fact that the midrange is wired out-of-phase. This obviously isn't helped by the fact that they have such an overlap. This doesn't happen using the Murphy crossover.

These are just initial observations, I need to get them set up properly and get down to some serious listening...

Well done!

Lee,

Thanks a lot for an interesting project. Well done and well written.Those speakers look better – and happier – now. Great photos as well. I’ve got some L100s myself which I want to give an x-over upgrade – while at the same time keeping them “original”. You project is an encouragement to this – thanks again.